Process Utilities

What are process utilities?Simple flowsheet: flammable and volatile feed/product

To run the process safely, we need:cooling water, steam, N2, compressed air, electricity

Cooling water systems Three systems normally used are:1). Once through2). Open evaporative recirculating3). Closed non-evaporative recirculatingOnce through systems Cooling water passes through the heat exchanger once. Once through systems can be used when plenty of cheap cool water is available and adequate facilities for disposal of warm water exist.Advantages: No cooling tower system; No water treatingDisadvantages: CorrosionFoulingWaste of waterThermal pollution of river

Open evaporative recirculating systems Cooling towers evaporate about 1% water. Water is reused after make up.Advantages:Less water requiredEnhanced corrosion control feasibleDisadvantages: Higher capital cost than once throughLarge cooling towers may be unacceptableSystem purge may pose environmental problems

Natural draft wet cooling towers

Crossflow type

Mechanical draft cooling towers

Counterflow type

Closed nonevaporative recirculating systems Cooling water is cooled in a secondary (air) heat exchanger. No evaporate, no makeup.Advantages:Water remains cleanCooling water temperature above 100oC is possibleDisadvantages: High capital costLimited by air temperatureOpen evaporative systems are usually used.

Basic calculations for open evaporative recirculating cooling water systems Evaporative rate:(m3/hr)T: temperature difference between feed and return water from cooling tower (oC); F: circulation rate (m3/hr)Windage loss, W: due to liquid entrainment, normally specified by tower manufacturer,0.01% of circulation for modern units, 0.2% for old unitsPurge and BlowdownLiquid water loss other than windage loss is termed Total Purge (P).P = B + ILB: blowdown, to limit solid build up, IL: leaksMake upM = E + W + P = E + W + B + IL

Concentration factor (CF)Evaporation increases the concentration of solid in the circulation water.

Typically, markers for X are magnesium or chlorine ions.

Calculation of make up and blowdown ratesMass balance on the marker (X):MXin = (P + W)Xout = (M - E)XoutHence

Since

Therefore, higher CF gives lower M and B.

Example:Circulation rate = 1000 m3/hrTw to tower = 30oCTw from tower = 20oC m3/hr

Make up rate at different concentration factors:

CF1.21.52.03.04.0M (m3/hr)85.842.928.621.519.1

System half life This is the time taken for the concentration of a soluble component (e.g. additive to control corrosion) to halve its initial concentration.

The above is obtained from the mass balance of the additive.

When t = t1/2, C(t) = 0.5C(0),

Steam DistributionSteam is used as a medium for transferring and transporting energy.Heating by steam condensation (heat exchanger)Mechanical work done by steam expansion (through turbine)Energy stored by latent heat and pressureTherefore, transport of steam = energy transportIn plant environment, typically a steam system includes:Central steam boilerSteam main circuit around plantHeat exchangers for process heatingCondensate return line to boilerIssues for steam distributionDistribution pressurePipe expansionHeat lossCondensate/air removal

Distribution pressureHigh pressureAdvantages: smaller mainslow installation costless insulation requiredDisadvantages: high pressure heat exchanger equipment or local pressure reduction valves requireddifficult to recover low grade heat (low temperature) as regenerated steam The above are reversed for lower pressure distribution

Pipe expansionDifference in pipe dimension when in use and when not in useExpansion allowance requiredExpansion fittings: Full loopHorse shoeBellowsSliding joint

Heat loss preventionSteam is hotter than surroundings, therefore heat loss is inevitable.Lagging is used to prevent heat loss (see Heat Transfer lecture notes)Typically lagged pipe heat loss 5-10% of that from bare pipe

Condensate/air removal (steam traps)Condensate collects at low points in pipe system. If not removed, pipe network will eventually be liquid filled.Water hammer: Fast moving gas meets slow moving slug of liquid resulting in rapid vibration of pipe work.Condensate accumulation controlled by deliberate slops in pipe work with intermittent drain points.Drain points are known as Steam traps.

Types of steam traps1. Thermostatic steam trapsBased on temperature difference between steam and condensate.Liquid expansion steam trapBimetallic steam trapBalanced pressure steam trap2. Mechanical float steam trapBased on density difference between steam and condensate.

Liquid expansion steam trapBimetallic steam trapMechanic float steam trap(www.spiraxsarco.com/resources/steam-engineering-tutorials)

Steam trap opening temperature selectionFor a thermostatic type steam trap, the trap open temperature should be selected according to the operating pressure and the corresponding steam saturation temperature. At a given operating pressure, the trap open temperature should be selected slightly lower than the steam saturation temperature at that pressure (the case of P2). In case P1, the steam trap open temperature is set too high and the steam trap will keep open even after all the condensate has drained. In case P3, the steam trap open temperature is set too low and the steam trap has to cool a long way before condensate is released. Therefore, condensate will collect in the steam distribution system.

Air removal

When the steam system is shut down, the pipe network is usually air filled.

Air can be purged using thermostatic steam traps because the temperature of air is lower than that of steam.

Water treatmentEvaporation in the cooling tower causes a build up of suspended/dissolved solids which can inhibit heat transfer by building up on heat exchanger surfaces - usually mould steel.Two problems in cooling water system:1). Foulingsilting/sedimentation (particles in source water, e.g. sand)scaling (precipitation of salts)biological growth (heat, oxygen, phosphates promote biological growth)2). CorrosionBoiler feed water needs also be treated to prevent fouling and corrosion.

Scale formationPrecipitation of the least soluble salts may occur, e.g. CaCO3, CaSO4.Ca++ + 2(HCO3)-- CaCO3 + H2O + CO2High concentration of Ca++ and SO4-- may also gives calcium sulphate scale (CaSO4).Scale impairs heat transfer efficiency and may increase pumping cost. With stainless steel, scaling may promote stress corrosion cracking.

Scale preventionHigher system purge to reduce CF at the expense of higher water/chemical costs.Soften makeup water: using external ion exchangers.Acid treatment to reduce [CO3--]: with water of medium to high CaCO3, i.e. > 800 mg/l, reducing the alkalinity to 20 - 40 mg/l will reduce CO3-- below the scaling level. H2SO4 or HCl are normally used.Scale inhibitors: modify crystal scale growthinorganic: polyphosphatesorganic: phosphorous compounds

Ion exchangeIons dissolved in the water are swapped with H+ and OH- ions held on different gel beads (ion exchanger resins).Beads are cross linked polymers containing ionic functional groups.The polymers are formed into porous particles to allow large contact area.Beads release H+ in exchange for metal ions.e.g.R-H+ + Ca2+ H+ + RCa+These beads form cationic exchange resin.Beads release OH- in exchange for other anions. These beads form anionic exchange resin.Typically resins have a weaker affinity for monovalent ions than for divalent ions (e.g. Ca2+)most salts are removed down to

Ion exchange

An ion exchanger can have two beds in series:cationic resin removes Ca2+ etc.anionic resin removes Cl- etc.The result from an ion exchanger is DEIONISED WATER, or demineralised.The capacity of an ion exchanger is not infinite and the supply of H+/ OH- can be exhausted.Regeneration of resins:i). Back wash with H2Oii). Cationic resin rinsed with excess H+, e.g. H2SO4iii). Anionic resin rinsed with excess OH-, e.g. NaOHOverall, ion exchangers are used for removing 1000 - 2000 mg/l of solute. Otherwise, regeneration becomes too frequent.Difference between boiler feedwater treatment and cooling water treatment:The requirement for boiler feed water treatment is stringent than for cooling water treatment.More hazardous operation (generating steam at high temperature and high pressure).Main problem concentrated in single unit operation (boiler).

(www.spiraxsarco.com/resources/steam-engineering-tutorials)

Removal of suspended solidsFlocculation larger particles larger terminal velocity (ut)SedimentationFinal filtrationFor very fine suspended particles (< 1m), it is expensive to filter them out.Small pore size = high Prapid clogging of filter coagulate to larger particles more easily removed.

CoagulationAddition of Al2(SO4)3 (coagulant) forms a precipitate with Ca(HCO3)2 in water.Al2(SO4)3+3Ca(HCO3)2 2Al(OH)3+3CaSO4+6CO2Al(OH)3 forms small particles which bind to the suspended solids, forming flocks.Other matters (bacteria, organics, etc.) become enmeshed in flockspH 5.8-7.4 required to ensure precipitate of Al(OH)3other coagulant available, e.g. FeSO4, FeCl3

SedimentationEasiest way of removing flocs, settling out of flocsRelies on terminal velocity of particles.If ut is still small, flocculation aids are used to form larger particles.Flocculation aids:long polymer molecules that stitch between flocs larger particles lager ut shorter settling timeFiltrationIn typical cake filter, filter cake is responsible for filtering, i.e. coarse particles produce filter medium. For very fine particles, the filter cake produced has very small pores.liquid cannot penetrate easilyfilter blocks rapidly use deep bed filtrationLiquid is passed through a bed of sand. Eventually fine particles make contact with solid surface and attach themselves.